A crucial element in sustainable energy supply is energy storage. Batteries are relatively expensive and have short life-span, and they can consume valuable natural resources. They are used for very small installations, when no other choice is available. In large-scale power generation, usually only concentrating solar power (CSP) plants have energy storage because they can be easily charged with solar heat. In large-scale CSP plants, molten salt is used (e.g., at Andasol in Spain and at Crescent Dunes in Nevada) for storage of heat from very large solar concentrating installations.
Photovoltaic (PV) collectors are cheap and easy to install, and require less monitoring and maintenance than CSP plants. This is important for small and medium size installations.
Wide-temperature range (e.g., gradient) heat storage is a very efficient way of storing energy. A trilateral cycle is a very efficient cycle for charging and discharging a gradient heat storage device or unit. Trilateral cycles have (1) a high-temperature, high-pressure, isobaric gradient process, (2) a gas pressure-changing process, (3) a low-temperature, isothermal, low-temperature isobaric process and (4) a liquid pressure-changing process.
Working fluids for trilateral cycles can be classified in three categories, depending on the slope of the saturation vapor curve on a TS diagram. The fluids may be classified as wet, isentropic or dry. Water and ammonia are wet fluids with a negative dT/ds slope, trichlorofluoromethane (R11) is isentropic with a vertical slope (infinite dT/ds), and some organic Rankine cycle (ORC) fluids such as pentane are considered dry and have a positive dT/ds slope.
Traditionally, a trilateral cycle may be made in one of three different ways, but each has at least one disadvantage. For example, disadvantages of a transcritical trilateral cycle (e.g., using carbon dioxide as a working fluid) include nonlinearity and very high pressures. Disadvantages of an ORC cycle is the fixed top temperature and the narrow temperature range. Wet subcritical trilateral flash cycles are not reversible.
A thermoelectric energy storage device that uses environmental air as the low-temperature heat source/sink is bulky and has low efficiency because of losses in the heat transfer to the air. In that case, a combined cycle with both high- and low-temperature heat storage may be useful. If abundant water is available, then a single trilateral cycle is efficient. However, this may not be the case in a city or a desert.
Increasing the efficiency of a Rankine cycle by preheating the feed water with steam tap is a 100-year-old technology having different names in the literature. It is called feed water heating or preheating in U.S. Pat. Nos. 938,309 and 8,572,968, steam tap in U.S. Pat. No. 1,781,368, steam-bleed in U.S. Pat. No. 1,615,003, regenerative Rankine or Rankine recuperator in U.S. Pat. No. 5,421,157, Carnotization of steam cycle in U.S. Pat. No. 2,964,910, multi-stage heat recovery in U.S. Pat. No. 3,681,920, and partly expanded steam in U.S. Pat. No. 3,979,914. A Rankine cycle with steam tap is not reversible, and has a fixed relation between heat absorbed from an isothermal source and heat rejected to an isothermal sink.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.